Direction finding is the establishment of the direction from which a received signal was transmitted. By combining the directional information from two or more suitably spaced receivers, the source of a transmission can be determined through the process of triangulation. Direction finding often requires an antenna that is directional by being more sensitive in certain directions than in others and a number of antennas exhibit this property. Direction finding antennas have been used in numerous military and civilian applications such as aircraft navigation, disaster response, search and rescue, reconnaissance, signals intelligence, airborne or ground Identification of Friend or Foe (IFF) systems and law enforcement.
While direction-finding antennas have found many useful applications, they can still suffer from a number of disadvantages, shortcomings and limitations. One significant disadvantage with current direction-finding antenna systems is the antenna spacing problem typically found in antenna arrays with 4 or 5 vertical monopole or dipole antennas. It is possible to determine the Angle of Arrival by measuring the time delay or phase difference (Interferometric technique) between all antennas in the array, but in interferometric direction-finding systems, antenna spacing is dependent upon the longest RF wavelength that is being detected. In the lower VHF band, such antenna spacing can become prohibitively large. Also, in TDOA systems the antennas spacing is determined by the minimum detectable time difference and therefore the antenna spacing can again be prohibitive. Thus, there has been a long-felt need for a direction-finding antenna that reduces or eliminates antenna spacing problems, particularly in the lower VHF band.
Another disadvantage with current antenna array direction-finding systems is the relatively large footprint found when using four widely spaced antennas in an array, which, in turn, causes a number of operational and tactical problems such as unwanted detection by an adversary, as well as cumbersome and complex calibrated cable assemblies. Other disadvantages related to the excessive size and unwieldy nature of current direction-finding antenna arrays include the expense of multiple antennas, excessive weight and wind drag. Additionally, processing signals from a multiple antenna array also requires more costly and sophisticated electronics equipment.
Thus, there has been a long-felt need for direction-finding antenna systems with fewer antennas that are simpler, less costly, less cumbersome and able to avoid unwanted detection by adversaries through a reduction in antenna spacing. Up until now, this long-felt need for such direction-finding antenna apparatus has not been answered.